The present invention relates to a horizontal speed indicator for a rotary-wing aircraft, especially a helicopter, used when the aircraft is moving in all directions.
Such indicators are generally intended to equip search and rescue helicopters for hovering flight especially above the sea or alternatively for antisubmarine warfare. For this reason, by extension, they will also be known hereinafter as "hovering flight indicators".
Hitherto, hovering flight indicators have usually been of the electromechanical type. These indicators display the speed with respect to the ground (more succinctly known as the groundspeed), or horizontal speed, measured for example using a Doppler-effect radar. This speed is resolved along the X--X axis and Y--Y axis of the helicopter (where X--X represents the longitudinal axis of the aircraft and Y--Y is orthogonal to X--X) and its components Vx, Vy are each represented by a pointer, the two points forming a sort of cross wire (two pointers or wires intersecting at right angles).
However, such indicators already have a drawback insofar as the information they display can be interpreted in two ways. This is because in a conventional indicator marked with a cross at its center and where the vertical pointer (wire) moves laterally to indicate a lateral speed and the horizontal pointer indicates a longitudinal speed:
either the intersection of the pointers marks the end of the groundspeed vector, while the center of the indicator marks the origin of the vector (or the helicopter); PA1 or, conversely, the intersection of the pointers represents the zero reference of the groundspeed. PA1 a first sensor and a second sensor for the heading and the horizontal speed of the aircraft, respectively, PA1 means of processing the signals delivered by said first sensor and said second sensor, and PA1 means of displaying the processed signals, showing on a display screen:
So-called screen indicators of this type are also known, and they also use the above principle but can represent the current groundspeed in the form of a vector rather than using intersecting pointers. In this case, the instrument also gives a representation of space and can display the objective to be reached (ship in distress or shipwrecked for example). For this, stationary concentric marks centered on a central reference representing the helicopter indicate both distances and speed magnitudes.
Furthermore, the logic involved in providing on-screen displays tends to deviate from that employed in conventional indicators. This is because circular representations are, on the one hand, very cumbersome for transmitting to a screen and, on the other hand, because the movement of a reference point along a stationary scale is not consistent with one's perception of one's surroundings (in this sense, the altimeter for example is organized in a way which goes against that of the artificial horizon which has the advantage of instinctive analogy). For this reason it is difficult for an indicator of the above type to be integrated into an organization consistent with that of the artificial horizon where the vehicle is represented as stationary (for the pilot) in moving surroundings.
Furthermore, it would seem that for this last type of indicator, for reasons associated with the way in which it is determined, the current speed displayed always has a lag. Furthermore, with fixed scales, the fields of distance and of speed that can be displayed remain limited.